An Atwood's Machine involves a frictionless and massless one of these devices, and students are often asked to calculate the 
An Atwood's Machine involves a frictionless and massless one of these devices, and students are often asked to calculate the tension on the objects on either side of these. Van de Graaff generators create positive charge by running a belt through a metal and a plastic one of these, and the mechanical advantage of an ideal block and tackle system is equal to the number of these objects in the system. For 10 points, name this simple machine used to lift heavy masses, which is made by wrapping a string around a wheel.
An Atwood's Machine involves a frictionless and massless one of these devices, and students are often asked to calculate the tension on the objects on either side of these.
{Pulleys}